STRUCTURE OF THE EYE 287 



retina (sensitive layer). The sclerotic is a firm membrane 

 composed of white fibrous tissue lined externally and 

 internally with a layer of endotheUum. The internal 

 endothelial layer contains a network of pigment cells 

 [lamina fusca). 



At the front of the eye the fibrous tissue of the sclerotic 

 becomes modified to form the transparent cornea. The 

 cornea has a smaller radius than the rest of the eye, and 

 therefore forms a projection upon what is otherwise an 

 almost perfect sphere. 



In the cornea five layers are recognised : (a) stratified 

 epithehum, continuous with the conjunctiva; (6) the 

 anterior elastic layer of Bowman; (c) the substantia 

 propria^this consists of laminse of connective tissue fibres 

 arranged parallel to the surface and separated by cell- 

 spaces or lacunae, in which he corpuscles ; [d) the posterior 

 elastic layer of Descemet; (e) endothehum. 



The cornea has no blood-vessels, its cells Ijeing nourished 

 by a flow of lymph from peripheral blood-vessels. The 

 surface of the cornea is kept clean by the tear-fluid secreted 

 from the lachrymal gland. 



The choroid is composed of three layers : (a) externally 

 the lamina suprachoroidea, which contains pigment-cells; 

 (6) the vascular layer, in which the blood-vessels form a 

 rich anastomosis ; (c) the membrane of Bruch. 



In the anterior part of the eye the choroid is modified 

 to form the ciliary glands and muscles and the iris. 



At the cihary glands the surface of the choroid is thrown 

 into folds (ciliary processes), which afford attachment to 

 the suspensory ligament of the lens. The cihary glands 

 secrete aqueous humour. 



The cihary muscles will be described later in connection 

 with accommodation. 



The iris forms a diaphragm having a central aperture. 

 It is composed of three layers : (a) an anterior layer of 

 endothehum, continuous with the posterior layer of the 

 cornea; (6) a layer of fibrous connective tissue; (c) a 



